Recently, weight saving in a steel sheet for automobile has been promoted in view of environmental issue, and use of a cold rolled steel sheet having improved strength has been investigated for inner and outer panels of automobile. The cold rolled steel sheet for inner and outer panels of automobile is required to have excellent stretchability, dent resistance, surface precision, anti-secondary working embrittlement, anti-aging, and surface appearance, and a high strength cold rolled steel sheet having such characteristics and a tensile strength of 370 to 590 MPa is now strongly desired by automobile manufacturers.
Before now, for example, JP-A-5-78784 proposes a high strength cold rolled steel sheet having a tensile strength of 350 to 500 MPa, which comprises a Ti-bearing ultra-low carbon steel added with a large amount of solid solution hardening elements such as Mn, Cr, Si, or P.
JP-A-2001-207237 or JP-A-2002-322537 proposes a galvanized steel sheet (dual phase structure steel sheet: DP steel sheet) having a tensile strength of less than 500 MPa, which comprises 0.010 to 0.06% C, 0.5% or less Si, not less than 0.5% to less than 2.0% Mn, 0.20% or less P, 0.01% or less S, 0.005 to 0.10% Al, 0.005% or less N, 1.0% or less Cr, wherein (Mn+1.3 Cr) is 1.9 to 2.3%, and consists of ferrite phases and second phases (low temperature transformation phases) of 20% or less by area ratio containing martensite phases of 50% or more.
However, the high strength cold rolled steel sheet described in JP-A-5-78784 has poor anti-aging, bad surface appearance due to a large amount of Si causing a problem in plating, and poor anti-secondary working embrittlement due to a large amount of P.
On the other hand, the DP steel sheet described in JP-A-2001-207237 or JP-A-2002-322537 does not have such problems since it is strengthened by second phases, however, it was found from the inventor's supplementary examination that the steel sheet did not always have sufficient stretchability and therefore it was not always applicable to outer panels of automobile.